what he means is you need to dps one of the tanks past the 20% intervals before the 2nd double destruction hits. If you don't you'll have 2 dps taking double damage during the aoe/shield phase, and you'll also have a chance of that spire dropping on top of the boss (usually where the tank is standing)

Out dps it or anticipate it and make distance between storm's tank and dps before the spire comes down. You'll need extra heavy heals on the 2 dps with the double destruction buff, but it's possible to make it through.